Variable frequency oscillator



Oct. 25, 1949.

FIG.

CON TROL TUBE F. R. DENNIS 2,486365 VARIABLE FREQUENCY 05 C ILLATORFiled March 22, 1947- FREQUENCYMCP AT OUTPUT 9 l I l l l l I I I I I l oa a 4 s e 1 e 9 IO'II la' :3 :4

NEGATIVE ac. co/vTRm. voLrs 01v cam a 0/-' was va lNVENTOR F R. DENNISATTORNEY Patented Oct. 25,1949

2,486,265 VARIABLE FREQUENCY OSCILLATOR Fred R. Dennis, Lyndhurst, N. Jassignor to Bell Telephone Laboratories,

Incorporated, New

York, N. Y., a corporation of New York Application March 22, 1947,Serial No. 736,437

Claims. I

This invention relates to oscillation generators and particularly tomeans for modulating or varying the oscillation frequency thereof.

One of the objects of this invention is to provide frequency control ofor frequency modulating in an oscillating system.

Another object of this invention is to provide a relatively wide rangeof frequency deviation in a frequency modulated oscillator with smallamplitude modulation.

Another object of this invention is to obtain a relatively wide rangelinear sweep in a frequency modulated oscillator.

Oscillators having a frequency which can be continuously varied over aspecified band of frequencies by either mechanical or electronic meansare useful in many applications in the radio art. Such an oscillator mayform a part inductive reactance across the oscillatorfrequency-determining circuit. When the transconductance of themodulator control tube is varied, as by varying one of the elementvoltages thereof, the magnitude of the reactance across the oscillatortuning circuit may be varied. By

of a visual gain set for example, wherein the re--- quired scanningrange may be a relatively wide band of frequencies such as, for example,a range.

of 40 to 4,000 kilocycles per second. In order to 'scan up to such afrequency of 4 or more megacycles per second, the scanning oscillatormay be 7 operated at a much higher frequency and then modulated downwith an associated fixed frequency oscillator fed into a common mixertype modulator tube, for example. In accordance with this invention, anoscillator of the reactance tube modulator type is provided in asimplified, form which gives an improvement not only with megacycles persecond with a linear scanning.

range of about 5 megacycles per second or more. For this purpose, theoscillator circuit may comprise as an elment thereof an artificial halfwavelength section of transmission line used in com bination with areactance tube modulator for obtaining the wide range linear sweep.

One of the most practical ways of obtaining frequency modulation isthrough the use of a reactance tube modulator. In this arrangement, theplate-cathode circuit of the modulator control tube may be connectedacross a portion of the oscillator frequency-determining circuit andmade to appear either as a capacitive or inductive reactance by excitingthe modulator grid with a voltage which either leads or lags theoscillator voltage by 90 degrees. The leading or lagging grid voltagecauses a corresponding leading or lagging plate current, and theplatecathode circuit may appear as a capacitive or applying an audio orother modulating voltage to one of the control tube electrodes, thetransconductance and hence the frequency of the oscillations generatedmay be varied at an audio or other rate. When properly designed andoperated, the reactance tube modulator is capable of producing largeamounts of frequency deviation, and of giving linear frequencymodulation. The type of phase-shifting circuit which is used to give agrid voltage which is in phase quadrature with the radio frequencyoscillator voltage at the plate of the modulator tube, is a feature ofspecial interest in accordance with this invention, in that it comprisesthe phase-shift networks of the oscillator itself, instead of a separatediscriminator or phase-shifting means.

The oscillator circuit per se may comprise a socalled phase-shift typeoscillator which may utilize a plurality of ladder type phase-shiftnetworks in the feedback circuit of the oscillator tube, and anauxiliary control path may be provided in the form of a reactance tubewhich is used to modulate or vary the frequency of the phase-shift typeoscillation generator, the phaseshifted input for the reactance tubebeing obtained from the frequency sensitive networks in the oscillatorfeedback circuit, rather than from a separate discriminator. A reactancetype control tube modulator rather than a resistance type control tubemodulator is employed for frequency control purposes, and the -degree,or integral multiple thereof, phase-shifted input for the reactancecontrol tube modulator is obtained directly from the oscillator feedbacknetwork, rather than from a separate QO-degree phaseshiftingdiscriminator usually associated with the reactance tube as in prior artarrangements. Accordingly, the phase-shifted input to the reactance tubeis, in accordance with this invention, obtained from a part of theoscillating circuit in order to obtain a simple form of wide rangefrequency modulated oscillator which may be used'as a scanningoscillator, for example, for obtaining a wide range linear sweepfrequency variation. The previously known form of separate QO-degreephase-shift discriminator arrangement is illustrated by such patents asChireix et a1. United States Patent 2,076,264 dated April 6, 1937,Crosby United States Patent 2,383,858

dated August 28, 1945, British Patent 564,504 of September 29, 1944, andBritish Patent 570,392 of July 5, 1945. Another form of prior artfrequency modulator utilizes an impedance control quency oscillationgenerator circuit, in accord-" ance with this invention;

Fig.2 is a graph illustrating an example of the variable frequencycharacteristics of an oscillator circuit of the type illustrated in Fig.1.

Referring to the drawing, Fig. 1 is a diagram illustrating a frequencymodulated oscillator circuit which may comprise an oscillator tube VIprovided with a main feedback circuit network system N whichregeneratively couples the output and input circuits of the oscillatortube VI. Also, there is'provided an auxiliary control path comprising afrequency control tube in the form of a reactance control tube V2 whichis connected with the half wave transmission line network system N. Asillustrated in Fig. 1, the oscillation generator part of the circuit ofFig. 1 is comprised mainly of the oscillator or power tube VIconstituting the gain circuit, and the feedback circuit network system Ncomprising a terminated plurality of tandem-connected networks couplingthe output or plate electrode circuit of the oscillator tube VI with theinput or control grid electrode circuit thereof. The control meanscomprises an auxiliary path including the tube V2 which is adapted tomodulate or vary the frequency of the oscillations generated inaccordance with the amplitude of the voltage which may be applied to thecontrol grid 3 of the reactance tube V2 from any suitable source, suchas the negative direct current bias voltage supplied from thepotentiometer PI and the alterhating current signal voltage suppliedfrom the audio signal source S. The output oscillations from the platecircuit of the oscillator tube VI may be taken off at the outputterminals 9 and'5,

labeled output in Fig. 1, and may be supplied to any desired utilizationor load circuit through any suitable limiter or buffer tube, orthrough'a mixer modulator tube for heterodyne purposes for example.

' As illustrated in Fig. 1, the oscillator tube VI may'comprise aconventional pentode VI having a cathode electrode I which may beheated'by a cathode heater filament 2, a control grid electrode 3 whichmay be connected to ground 5 through a grid resistor RI and which may beconnected to the grid terminal G of the network system N through acondenser Cl, a screen grid electrode 6 which may be connected to ground5 through a condenser C3 and which may be connected through a resistorR3 to the posi-' tive terminal of a suitable power supply source I0, asuppressor grid electrode I which may be connected in a conventionalmanner to the shell of the oscillator tube VI and to ground 5, and ananode or plate electrode 8 which may 4 be connected through a condenserCI3 to the output terminal 9 and which may be connected through aresistor R23 to the plate terminal P of the network system N. Forobtaining plate supply voltage for the plate electrode 8 of theoscillator tube VI, the connection may extend through the seriesconnected windings LI and L2 of the network system N and throughterminating resistor R5 and isolating resistor R4 to the positiveterminal of the power supply source I0. Condensers C8 and C9 may beprovided between the ground 5 and the positive terminal line of thepower supply source ID. The cathode electrode I of the oscillator tubeVI may be connected to ground 5 through a cathode resistor R2 which maybe shunted by a by-pass condenser C2. The cathode heater filament 2 maybe energized by any suitable supply source (not shown). While theoscillator tube VI has been illustrated in Fig. l as comprising a singlepentode VI, it will be understood that any-suitable oscillator tubesystem may be utilized to provide the ,u or gain source for theoscillations generated in the circuit.

The network system N, as illustrated in Fig. 1, is disposed in the mainfeedback circuit of the oscillator tube VI and regeneratively couplesthe output and input circuits thereof, the output or plate electrode 8of the oscillator tube VI being connected with the input terminal P ofthe multistage network N, and the'output terminal G of the network Nbeing connected with the input or control grid electrode 3 of-theoscillator tube VI through the condenser CI. The arrangement of thenetwork system N is such that it provides a total phase shift ofsubstantially 180 degrees, or an integral multiple thereof, in thefeedback or B circuit of the oscillator tube VI and operates as afrequency-determining means for controlling the frequency of theoscillations generated in the oscillator circuit comprising the tube VIand the network system N, the frequency of oscillations being determinedmainly by the values of the component elements of the network system Nwhich may comprise inductance and capacitance elements as particularlyillustrated in Fig. '1, or other suitable phase-shifting elements thatyield a total phase shift of nominally 180 degrees, or an integermultiple thereof, in the feedback networksystem N.

As particularly illustrated inFig. 1, the network system N comprises atwo-stage ladder type phase-shifting network system consisting of theseries inductance elements LI and L2 and the shunt capacitance elementsC5, C6 and C1, thus providing in effect 'a. pair of tandem-connectednetworks at the sideswof "thejunction point K between the inductanceelements LI" and L2; each section of the pair of networks yielding aphase shift of nominally degrees. :A condenser C4 may be providedbetween the terminal J of the network system N'and the junction point Kreferred to, that is between the inductance windings LI and L2. Thesubstantially QO-degree'or quadrature phase shift that is providedbetween the terminal J.and eithertheinphtterminal P 'or the outputterminal Gofthe-oscillator feedback network system N may be"'utilized,in cooperation with the auxiliary control path including the reactancecontrol tube V2, for'obtaining a relatively wide rangelin'e'ar frequencyvariation or modulationof'. the frequency of the oscillations generatedby the oscillator circuit comprising the oscillator tube VI and thefeedback network systemNm v As illustrated in Fig. 1, an auxiliarycontrol path including the control tube V2 may be provided formodulating or varying the frequency of the oscillations generated by theoscillator tube Vi. The control tube V2 may be, as illustrated in Fig.1, a pentode having a cathode electrode l which may be connected toground 5 through a cathode resistor R8 shunted, if desired, by acondenser 02!, a cathode heater 2 which may be energized by any suitablepower supply source (not shown), a control grid electrode 3 which may beconnected through a grid resistor Rlil to the junction terminal J of theoscillator network system N, a screen grid electrode 6 which may beconnected through a condenser CI2 to ground 5 and which may be connectedthrough a resistor R9 to the positive terminal of the power supplysource ID, a suppressor grid electrode 7 which may be connected in aconventional manner to ground 5, and an anode or plate electrode 8,which may be connected through a resistor R28 to the input or plateterminal P of the oscillator network system N. The control gridelectrode 3 of the reactance tube V2 may also be connected through theresistor RIB and a resistorR'I to the resistance potentiometer PI, andalso to the modulating signal source S through a condenser CM, and toground 5 through a condenser Cl l.

The resistance potentiometer Pl may be energized by a suitable directcurrent power supply source H having its positive terminal connected toground 5 and to one end of the potentiometer resistance PI, and havingits negative terminal connected to the other end or the potentiometerresistance PI. The potenttiometer Pi may be utilized to provide anegative direct current bias input voltage to the control grid electrode3 of the reactance tube V2, the magnitude of the voltage being madeadjustable by variation of an adjustable tap l2 on the potentiometerresistance Pl in order to vary the frequency of the oscillationgenerator or to provide for a center frequency adjustment thereof. Thesignal source S may be any suitable modulating source such as, forexample, a 60-cyc1e sine wave or triangular wave source adapted to varythe frequency of the oscillation generator V! in accordance with theamplitude of the voltage applied by the signal source S to the controlgrid electrode 3 of the reactance control tube V2. The control tube V2reflects a reactance into the oscillating circuit which is a function ofthe control voltage applied to the control grid electrode 3 of thereactance tube V2 through the resistor R1, provided such control voltagehas a 90-degree phase shift with respect to the voltage of the platecircuit electrode 8 of the oscillator tube V! Such a grid quadraturevoltage may be obtained, in accordance with this invention, at themid-branch of the line windings Ll and L2, as illustrated in Fig. 1, andmay be taken ofi at the junction terminal J of the network system N.

As illustrated in Fig. 1, the reactance elements Ll, L2, C5, C6 and C1of the two-section network system N are connected at one end terminal Pthereof to the output or plate circuit electrode 8 of the oscillatortube VI and comprise the equivalent of a half wavelength or ISO-degreephase-shift section of transmission line. Since a total phase shift ofsubstantially 180 degrees occurs in the line section N at the operatingfrequency of the oscillator VI, the oscillatory loop circuit may becompleted by connecting the other end terminal G of the network system Nto the control grid electrode 3 of the oscillator tube VI. If R0designates the line impedance at the operating frequency ,f, thefollowing relations obtain: 5,5

fc= where fc represents cut-0d frequency.

L1=L2=- Il'fC 1 27rfCC5 I For design purposes, reasonable values. forcapacitance may be assumed for the end condensers C5 and Cl, and thenthe values for the remaining constants of the network system N may becomputed. As an illustrative example, if 30 microlarads is chosen as thevalue of the condensers C5 and Cl, for a frequency f of about 30megacycles per second, the inductance value of Li or L2 comes out about0.94 microhenry, which gives about ohms for R0 which is the optimumvalue for terminating resistor R5. While the use of the resistor R8 inthe circuit of the cathode electrode l of the reactance tube V2 somewhatdecreases the total frequency variation obtainable, the linearity of thefrequency variation is improved thereby. Where the supply voltageapplied to the screen grid electrode 6 of the reactance tube V2 from thesupply source I0 is maintained of constant value by means of a suitablevoltage regulator tube such as by a VR-l05-30 tube (not shown), thereresults a slightly greater total frequency variation with less controlvoltage variation applied to the control grid electrode 3, but theseries screen grid type of supply voltage feed, as illustrated in Fig.1, may be utilized to give increased linearity.

The oscillator loop circuit gain represented by the tube Vi may be madeof a value not greatly exceeding that required to maintain stableoscillation over the entire control range for maximum' frequencydeviation, as provided by the control tube V2, by adjustment ofcathoderesistorr R2. The condensers C5, C6 and C! of the network systemN may be adjusted to the proper capacitance values in order to give anapproximately constant amplitude of output volt-. age at the outputterminals 5 and 9 over the sweep frequency range. Also, the outputamplitude may be made to either increase or decrease its magnitude withfrequency by proper adjustment of the condensers C5, C5 and CI, but withsome reaction on the extent of the sweep range and its linearity. Theoutput amplitude and the sweep range can be controlled to some extent byvariation of terminating resistor R5. The circuit of Fig. 1 may beprovided with some negative feedback for the modulating frequencythrough the cathode resistor R8 by making the capacitance of thecondenser C2l only enough to by-pass the high frequency of oscillation.

As an illustrative example for an oscillator circuit constructedsubstantially in accordance with the circuit of Fig. 1 and having anoutput linear sweep frequency range of approximately 25 to 30 or moremega'cycles per second, the component elements thereof may be made tohave the following values: the power supply source lll may supplya'voltage of about +250 volts, or other suitable value. The negativebias supply source II for the potentiometer PI may supply a bias voltageof about volts direct current, or other suitable value. The modulatingsweep signal source S may be, for example, a 60-cycle triangu lar wavehaving a suitable amplitude of voltage to modulate the control tube V2.The oscillator tube Vi and the reactance tube V2 may be conventional6AG7 vacuum tubes, or other suitable electron discharge devices. Theinductance windings LI and L2 may be equal inductance retardation coilshaving about 9 microhenries of inductance value for each, or othersuitable inductance value to suit the phase shift and impedancerequirements. The condenser 04 may have a capacitance value of about 500micromicrofarads, and the condensers C5, C6 and C7 about 5 to 20micromicrofarads each, and the remaining condensers may have capacitancevalues expressed in micromicroiarads about as follows: condenser Cl=500,02:5,000, C3=500, C8 and 09:5,000, Cll=500, Cl2=500, CE3=25, CM=4microfarads. The resistors may have resistance values expressed in ohmsabout as follows: Rl=10,000, R2=150, R3=33,000, R4 if used: 1,000,R5=135, RT=10,000, R8=47, R9=33,000, Rl9=33, R=l00, R23=33, Pi=l0,000.It will be understood that the resistance and capacitance and inductancevalues given are illustrative, and that other values may be used inaccordance with the requirements of the particular operating conditions.

Fig. 2 is a graph illustrating a typical plot of the variation in outputfrequency as expressed in megacycles per second at the output terminals9 and 5 of the circuit illustrated in Fig. 1, as a function of change inthe value of the direct current control voltage supplied to the controlgrid electrode 3 of thecontroltubeV2 bythe potentiometer Pl associatedwith the negative bias voltage supply source ll of Fig.1. As illustratedin Fig. 2, a substantially linear frequency characteristic may beattained over a relatively wide range of frequencies as produced by thevariable frequency oscillator circuit illustrated in Fig. 1. As shown inFig. 2, the frequency decreases with increasing control voltage values,but should it be desired to have the frequency increase with increasingcontrol voltage value, the plate electrode 8 of the control tube V2 maybe connected to the output terminal G of the network system N, insteadof to the input terminal P thereof, as now shown in Fig. 1. Asillustrated in Fig. 2, a linear scanning range of well over 4 megacyclesper second may be provided by the -megacycle circuit of Fig. l, withrelatively low amplitude modulation and an output amplitude variation ofnot over decibel over the scanning range.

While the sweep oscillator circuit as illustrated in Fig. 1 has beenparticularly described for operation at a frequency in the vicinity of30 megacycles per second with a frequency modulation range of about '7megacycles per second of which over 4 megacycles per second is linearwith respect to variation in the control voltage applied to the controlgrid electrode 3 of the reactance control tube V2, the circuit may bemade to operate at other frequencies with stable and reliable operationand with results in general closely duplicating those obtained by the30-megacycle per second oscillator referred to which had a network lineN of about 100 ohms impedance. As an illustrative example, theoscillator frequency of the circuit of Fig. 1 may be made to be in thevicinity of one megacycle per second for example with a network line Nof relatively high impedance of the order of 3,000 ohms for example, orwith a network line N of relatively low impedance of the order of ohmsfor example. The circuits employed may be substantially the same as thatillustrated in Fig. 1, with suitable changes provided in the values ofthe constants of the network line N, and in the couplings thereto toinsure the application of the correct operating voltages to the grids ofthe oscillator and control tubes VI and V2. A somewhat greater frequencyspread and better linearity may be more readily obtainable with a lowcapacitance network line N than with a high capacitance line N. It willbe understood that increased frequency variation may be obtained whenthe circuit of Fig. 1 is operated without a strict linearity relationbetween the oscillation frequency and the value of control voltagevariation applied to the control grid electrode 3 of the reactancecontrol tube V2. The degree of linearity of the oscillation frequency,the frequency spread and the constancy of oscillator output voltage arerather critical functions of the amount of quadrature voltage fed fromthe network N to the control grid 3 of the control tube V2, and of thevariations of the capacitances C5, C6 and Cl, particularly theterminating capacitance Cl. Variations up to about :20 per cent in theresistance of the line terminating resistor R5 may not greatly affectthe result.

It will be understood that oscillators of the type illustrated in Fig. 1may be made to operate in practically any frequency band. For the lowfrequency bands, the phase shift elements of the network system N maycomprise resistance and capacitance elements arranged in suitablenetworks N. For the higher frequency bands conductor core coils orcoaxial circuit elements for example may be used.

Although this invention has been described and illustrated in relationto specific arrangements, it is to be understood that it is capable ofapplication in other organizations and is therefore not to be limited tothe particular embodiments disclosed.

What is claimed is:

1. A generator of electrical oscillations comprising an electrondischarge device having input and output circuits means forregeneratively coupling said output circuit with said input circuitcomprising a plurality of phase-shifting networks connected in tandemand constituting means for yielding a total phase shift thereincorrespondin to one of the values of substantially degrees and anintegral multiple thereof at the operating frequency of saidoscillations, and frequency controlling means including an auxiliaryvariable reactance controlled transmission path for introducing variablereactance into said phase-shifting networks and thereby changing saidfrequency of said oscillations said variable reactance path beingoperatively connected between two connection points on saidphaseshifting networks the phase shift between which corresponds to oneof the values of substantially 90 degrees and an integral multiplethereof at said operating frequency.

2. A generator of electrical oscillations comprising an electrondischarge device having input and output circuits, means forregeneratively coupling said output circuit with said input circuitcomprising a plurality of phase-shiftin networks connected in tandem andconstituting means for phase shift between which corresponds to one ofthe values of substantially 90 degrees and a multiple thereof at saidoperating frequency, said path including an electronic reactance controldevice having its imput and output electrodes individually connectedwith said two connection points on said -p'l'iase-s'hifting networks,and means for supplying a variable potential to one of said electrodesof said control device forv changing said frequency of said oscillationsin accordance with the magnitude of said variable potential.

3. A- generator of electrical oscillations comprising an electrondischarge device having input and output circuits, means forregeneratively coupling said output circuit with said input circuitcomprising a plurality of phase-shifting networks constituting means foryielding a total phase shift therein correspondin to one of the valuesof substantially 180 degrees and a multiple thereof at the operatingfrequency of said oscillations, and means including an auxiliarytransmission path for changing said frequency of said oscillations, saidpath being operatively connected between two connection points on saidphaseshifting networks the phase shift between which corresponds to oneof the values of substantially 90 degrees and a multiple thereof at saidoperating frequency, said path including an electronic reactance tubehaving a grid electrode and a plate electrode, said grid and plateelectrodes being individually connected with said two connection pointson said phase-shifting networks, and means supplying a variablepotential to said grid electrode of said electron tube for changing saidfrequency of said oscillations in accordance with the magnitude of saidvariable potential.

4. A generator of electrical oscillations comprisin an electrondischarge device having input and output circuits, means forregeneratively coupling said output circuit with said input circuitcomprising a plurality of phase-shifting networks connected in tandemand constituting means for yielding a total phase shift thereincorresponding to one of the values of substantially 180 degrees and amultiple thereof at the operating frequency of said oscillations, andmeans including an auxiliary transmission path for changing saidfrequency of said oscillations, said path being operatively connectedbetween two connection points on said phase-shifting networks the phaseshift between which corresponds to one of the values of substantially 90degrees and a multiple thereof at said operating frequency, said pathincluding an electronic reactance tube having a grid electrode and aplate electrode, said grid and plate electrodes being individuallyconnected with said two connection points on said phase-shiftingnetworks, and means supplyin a variable potential to said grid electrodeof said electron tube for changing said frequency of said oscillationsin accordance with the magnitude of said variable potential, saidlast-mentioned means comprising a source of 10 alternating currentsignal potential connected with said grid electrode for modulating saidfre-; quency of said oscillations in accordance with the amplitude ofsaid variable signal potential,

1 and a source of negative direct current bias potential connected withsaid grid electrode for adjustin for the center frequency position ofsaid frequency of said oscillations.

5. A generator of electrical oscillations comprising an electronicsource of gain having input and output circuits, feedback circuit meansfor regeneratively coupling said output circuit of said electronicsource with said input circuit thereof and comprising a pair ofphase-shifting networks connected in tandem and constituting means foryielding a phase shift of substantially 99 degrees in each of said pairof networks at theoperatin frequency of said oscillations, meansincluding a reactance control tube having its input and outputelectrode-s connected across only one of said -degree phase-shiftnetworks for changing said frequency of said oscillations in accordancewith the magnitude of a control potential supplied to said inputelectrode of said control tube.

6. A generator of electrical oscillations comprising an electronicsource of gain having input and output circuits, feedback circuit meansfor regeneratively coupling said output circuit of said electronicsource with said input circuit thereof and comprising a pair ofphase-shifting networks constituting means for yielding a phase shift ofsubstantially 90 degrees in each of said pair of networks at theoperating frequency of said oscillations, means including a reactancecontrol tube having its input and output electrodes connected acrossonly one of said QO-degree phase-shift networks for changing saidfrequency of said oscillations in accordance with the magnitude of acontrol potential supplied to said input electrode of said control tube,said input electrode of said control tube being connected to thejunction connection between said pair of Bil-degree phaseshift networks,and said output electrode of said control tube being connected to theother or opposite end of one of said pair of QO-degree phaseshiftnetworks.

'7. A generator of electrical oscillations comprising an electronicsource of gain having input and output circuits, feedback circuit meansfor regeneratively coupling said output circuit of said electronicsource with said input circuit thereof and comprising a pair ofphase-shifting networks connected in tandem and constituting means foryielding a phase shift of substantially 90 degrees in each of said pairof networks at the operating frequency of said oscillations meansincluding a reactance control tube having its input and outputelectrodes connected across only one of said 90-degree phase-shiftnetworks for changing said frequency of said oscillations in accordancewith the magnitude of a control potential supplied to said inputelectrode of said control tube, said input electrode of said controltube being connected to the junction connection between said pair of90-degree phase-shift networks, and said output electrode of saidcontrol tube being connected to the other or opposite end of one of saidpair of QO-degree phase-shift networks, said other or opposite end ofsaid one of said pair of networks being the end connected with saidoutput circuit of said source of gain.

8. An oscillation generator in accordance with claim 7 wherein said pairof networks comprises 11 ladder type networks having inductive seriesarms and capacitive shunt arms.

9. A generator of variable frequency oscillations comprising anelectronic source of gain, a feedback circuit coupling the output withthe input of said source of gain, said feedback circuit comprising afrequency sensitive phase-shift network system having a plurality ofphase-shift sections, and means including a reactance control tubehaving its input and output electrodes connected with said networksystem in said feedback circuit for changing said frequency of saidoscillations in accordance with the magnitude of control potentialapplied to said input electrode of said control tube, said inputelectrode of said control tube being connected to a connection point insaid feedback network system having a substantially QO-degree phaseshift with respect to the connection point where said output electrodeof said control tube is connected to said network system.

10. A generator of variable frequency oscillations comprising anelectronic source of gain, a feedback circuit coupling the output withthe input of said source of gain, said feedback circuit comprising afrequency sensitive phase-shift network system having a plurality ofphase-shift sections, and means including a reactance control tubehaving its input and output electrodes connected with said networksystem in said feedback circuit for changing said frequency of saidoscillations in accordanc with the magnitude of control potentialapplied to said input electrode of REFERENCES CITED The followingreferences are of record in the file of this'patent:

UNITED STATES PATENTS Number Name Date 1,442,781 Nichols Jan. 16, 19232,236,985 Bartelink Apr. 1, 1941 2,300,632 Poch Nov. 3, 1942 2,321,269Artzt June 8, 1943

